1. Field of the Invention
This invention relates to a vacuum interrupter used in an electric circuit of high power, for example, with an alternating current circuit, more particularly to a vacuum interrupter of an axial magnetic field appliance type in which a magnetic field is applied in a direction parallel to an axis of an electric arc-current flowing across a space between a pair of contact-electrodes within a vacuum envelope of the vacuum interrupter when the pair is engaged or disengaged (hereinafter, the magnetic field is referred to as an axial magnetic field), thus enhancing current interruption capability of the vacuum interrupter.
2. Description of the Prior Art
A vacuum interrupter of an axial magnetic field appliance type restricts an electric arc to a space between a pair of separated contact-electrodes and uniformly distributes the arc in the space with the axial magnetic field thereof, thus preventing any local overheating of the contact-electrodes to enhance the current interruption capability thereof.
A conventional vacuum interrupter of the axial magnetic field appliance type, as shown in FIG. 1, is known. The interrupter comprises an evacuated envelope 3 including an insulating cylinder 1 and a pair of metallic end plates 2 joined to the opposite ends of the insulating cylinder 1, and a pair of stationary and movable contact-electrodes 4 and 5 within the envelope 3 which are engaged or disengaged to close and open a circuit. The contact-electrodes 4 and 5, which are of a generally disc-shape, each made of Cu, Ag, or alloy thereof of high electrical conductivity. The contact-electrode 4 or 5 is mechanically and electrically connected to the inner end of a stationary or movable lead rod 6 or 7 which extends within the evacuated envelope 3 securing vacuity thereof through an aperture centrally defined in each metallic end plate 2.
Coil-electrodes 8 and 9, each of which creates the axial magnetic field, made of high electrical conductivity material are located spaced from each of the contact-electrodes 4 and 5 therebehind. The stationary and movable contact-electrodes 4 and 5 and the corresponding coil-electrodes 8 and 9 are mechanically connected at the center portion to each other by means of spacers 8a and 9a of high electrical resistivity material, while electrically connected near the outer peripheral portions to each other by means of high electrical conductors 8b and 9b.
The coil-electrodes 8 and 9, when the movable contact-electrode 4 is separated from the stationary contact-electrode 5, create a magnetic flux between the contact-electrodes 4 and 5 by a circular current flowing through the coil-electrodes 8 and 9. The magnetic flux is changeable with time and passes the contact-electrodes 4 and 5 from the front surfaces thereof to the backsurfaces thereof or vice versa.
A metallic arc shield 10 of a generally circular cylinder is provided at the insulating cylinder 1, surrounding the contact-electrodes 4 and 5. Further, auxiliary metallic shields 11 are provided on the respective metallic end plates 2 near the opposite ends of the arc shield 10.
A metallic bellows 12 secures a hermetic sealing between the movable lead rod 7 which allows the movable contact-electrode 5 to engage or disengage from the stationary contact-electrode 4, and the corresponding metallic end plate 2.
The above-mentioned vacuum interrupter has a certain advantage in the aspect that the axial magnetic field which is applied to the space between the contact-electrodes 4 and 5 upon occurrence of a circuit interruption, enhances the current interruption capability.
However, according to the vacuum interrupter, the magnetic flux changeable with time which is created by the coil-electrodes 8 and 9 permeates through the stationary and movable contact-electrodes 4 and 5 of high electrical conductivity material, thus creating eddy current in the contact-electrodes 4 and 5 so as to reduce the axial magnetic field by the coil-electrodes 8 and 9.
For eliminating the disadvantages of the contact-electrodes 4 and 5, a pair of stationary and movable contact-electrodes 13 and 14, as shown in FIG. 2, are provided which are different from the contact-electrodes 4 and 5 in the aspect that a plurality of slits 15 are provided extending radially in the contact-electrodes 13 and 14 (Refer to U.S. Pat. No. 3,946,179A).
The contact-electrodes 13 and 14 have a certain advantage in the aspect that they eliminate eddy current in the contact-electrodes 4 and 5 of FIG. 1. However, the slits 15 considerably reduce the dielectric strength between the contact-electrodes 13 and 14 due to edges thereof and also the mechanical strength of the contact-electrodes 13 and 14.
Further, the slits 15 are filled up with deposited arcing products after the stationary and movable contact-electrodes 13 and 14 have interrupted large current at high voltage many times, namely, the contact-electrodes 13 and 14 are returned to a state wherein the contact-electrodes 13 and 14 lack slits. Consequently, the current interruption capability of the contact-electrodes 13 and 14 is considerably reduced.
For eliminating the disadvantages of the contact-electrodes 4, 5, 13 and 14 of FIGS. 1 and 2, a pair of stationary and movable contact-electrodes 16, as shown in FIG. 3, are provided which are different from the contact-electrodes 4 and 5 of FIG. 1 in the aspect that they are made of material of at most 40% IACS electrical conductivity, e.g., Be, Cu-W alloy or Ag-W alloy (refer to JP-57199126A, published on Dec. 7, 1982). According to the pair of contact-electrodes 16 disclosed as a prior art in the JP-57199126A, eddy current created in the contact-electrodes 16 are considerably reduced although the slits are not provided therein. However, if the contact-electrodes 16 are employed together with the coil-electrodes 8 and 9 disclosed in FIG. 1, when an electric arc A is located away from a contact-making portion 17 at the center portion of the contact-electrode 16 before the arc is distributed on the whole surface of the contact-electrode 16, depending upon any outer magnetic field and/or other affections, differences between values of branched arc currents i.sub.1, i.sub.2, i.sub.3 and i.sub.4 which, for example, flow in the contact-electrodes 16 from or to the coil-electrode 9 through the high electrical conductor 9b are spread because of lowered of electrical conductivity of the contact-electrodes 16, and further the branched arc current i.sub.1 is caused to be maximal and finally the branched arc currents i.sub.2, i.sub.3 and i.sub.4 are caused to be substantially zero. Consequently, the magnetic flux density of the located axial magnetic field applied by the branched arc current i.sub.1 through flowing the coil-electrodes 8 and 9 to the interelectrode gap is caused to be maximal at the location of the arc and uniformity of the magnetic flux density which is to be produced substantially over the interelectrode gap is considerably impaired. The current interruption capability of the contact-electrode 16 is considerably lowered.
Further, a temperature rising in the contact-electrodes 16 becomes more severe during a normal current flowing because of the low electrical conductivity of the contact-electrodes 16.